Mayor Bloomberg yesterday defended the federal decision not to cover cancer-stricken 9/11 first responders under the $2.7 billion Zadroga Act, even as the city’s top cop blasted the ruling.

“You don’t mess with science,” Bloomberg said testily when asked by reporters about the National Institute for Occupational Safety and Health’s ruling Tuesday that there isn’t a clear link between World Trade Center dust and cancer.

“And the standards that science requires are very different than the standards that politics require,” Bloomberg went on.

The latest finding from NIOSH means that cops, firefighters, ironworkers and others who have been diagnosed with cancer since 9/11 are unable to tap the federal funds for medical care set aside for them in the Zadroga Act, named for the late NYPD cop James Zadroga.

Police Commissioner Ray Kelly said he was “very disappointed” by the decision.

“Twenty-three officers killed on 9/11. We’ve had 49 illnesses that they contracted from Ground Zero or from the landfill in Staten Island. Based on the knowledge I have, 46 of those 49 died as a result of cancer,” he said. “So, at least from a layman’s view, it certainly looks like there is connectivity here.”

NIOSH had said it would look at the issue again next year.

But that didn’t keep some first responders from getting irate last night at a Queens town-hall meeting on the Zadroga fund.

“I think this is an injustice,” said retired NYPD Detective John Marshall, who suffers from throat cancer and who worked at Ground Zero for three months, starting on 9/11.